If the two stacked subs are in phase and correlated (which they would be a subwoofer frequencies), they would yield +6db in free field conditions.
In a real room, it might be close, but you would have to measure it.
However, the idea of "stacking" is a poor one, because greater spatial distribution will yield smoother (amplitude) response, while remaining (mostly) correlated due to the large wavelengths involved.
Get more, spread 'em.